Polycondensations lead polymers

Amino acids containing nucleobases like uracil and adenine, as well as imidazole, such as / -(uraciM-yl)-a-alanine, j5-(adenin-9-yl)-a-alanine, and j8-(imidazol-l-yl)-a-alanine, can also be polycondensed by CDI in aqueous imidazole buffer solution at pH 6.8 at 0 °C. The polycondensation leads to low conversion (yields of polymer 1% after four days), but pure polypeptides resulted from the reaction. Thus, compared to other alternative procedures for polycondensation, that using CDI proved to be the most effective 503... 

Decomplexation of the P(III) moiety of the boronhydride complex (10) by the base l,4-diazabicyclo[2.2.2]octane (DABCO) followed by an internal Staudin-ger reaction has been reported to yield the linear polymer (12). Hyperbranched polymers are obtained by using the same procedure but starting from the diphosphine (13). Treatment of (11) with NaNs leads to an intramolecular Staudinger polycondensation affording polymer (12) as well. Compound (13) has proven to be a useful starting material for the synthesis of dendrimers with phosphine end groups. ... 

Ring-opening polymerization generally yields high molecular weight (MW) polymers, whereas direct polycondensation leads to low MW products. 

A series of DTDPP-based copolymers with an alternating D-A sequence and homopolymer P32 were synthesized by the direct arylation polycondensation. The polymer was endowed with the common features of excellent ir-conjuga-tion and ideal planarity, which lead to remarkably low band gaps (1.22 eV). Photovoltaic properties of these polymers were not reported (Scheme 1.33). ... 

Using a mononuclear tetramine a polycondensation very similar to that used with BBB can occur which leads to a ladder polymer of high structural perfection (BBL) Figure 29.20). 

Another definition, taking into account polymerization conversion, has been more recently proposed.192 Perfect dendrimers present only terminal- and dendritic-type units and therefore have DB = 1, while linear polymers have DB = 0. Linear units do not contribute to branching and can be considered as structural defects present in hyperbranched polymers but not in dendrimers. For most hyperbranched polymers, nuclear magnetic resonance (NMR) spectroscopy determinations lead to DB values close to 0.5, that is, close to the theoretical value for randomly branched polymers. Slow monomer addition193 194 or polycondensations with nonequal reactivity of functional groups195 have been reported to yield polymers with higher DBs (0.6-0.66 range). 

Alkyl esters often show low reactivity for lipase-catalyzed transesterifications with alcohols. Therefore, it is difficult to obtain high molecular weight polyesters by lipase-catalyzed polycondensation of dialkyl esters with glycols. The molecular weight greatly improved by polymerization under vacuum to remove the formed alcohols, leading to a shift of equilibrium toward the product polymer the polyester with molecular weight of 2 x 10" was obtained by the lipase MM-catalyzed polymerization of sebacic acid and 1,4-butanediol in diphenyl ether or veratrole under reduced pressure. ... 

The problems associated with route B also have something to do with steric hindrance. Here the critical point is the steric demand of both monomer and chain end. Incoming monomer will only be connected to the chain end, if steric hindrance is not too high. Otherwise this process will be slowed down or even rendered impossible. Depending on the kind of polyreaction applied, this may lead to termination of the reactive chain end and/or to side reactions of the monomer, like loss of coupling functionality as in some polycondensations or auto-initiation specifically in radical polymerizations. From this discussion it can be extracted that the basic problems for both routes are incomplete coverage (route A) and low molecular weight dendronized polymer (route B). 

A further unusual feature of the matrix-dependent polycondensation lies in the character of the nucleobases themselves. Purine mononucleotides undergo polycondensation, in good yields, at complementary matrices consisting of pyrimidine polymers. However, the synthesis of pyrimidine oligonucleotides from their mononucleotides at purine matrices is not effective. This important fact means that a pyrimidine-rich matrix leads to a purine-rich nucleic acid, which is itself not suitable to act as a matrix. This phenomenon also occurs when matrices are used which contain both basic species, i.e., purines and pyrimidines. An increase in the amount of purine in a matrix leads to a clear decrease in its effectiveness (Inoue and Orgel, 1983). However, the authors note self-critically that the condensation agent used cannot be considered to be prebiotic in nature. 

Referring to the ADMET mechanism discussed previously in this chapter, it is evident that both intramolecular complexation as well as intermolecular re-bond formation can occur with respect to the metal carbene present on the monomer unit. If intramolecular complexation is favored, then a chelated complex, 12, can be formed that serves as a thermodynamic well in this reaction process. If this complex is sufficiently stable, then no further reaction occurs, and ADMET polymer condensation chemistry is obviated. If in fact the chelate complex is present in equilibrium with re complexation leading to a polycondensation route, then the net result is a reduction in the rate of polymerization as will be discussed later in this chapter. Finally, if 12 is not kinetically favored because of the distant nature of the metathesizing olefin bond, then its effect is minimal, and condensation polymerization proceeds efficiently. Keeping this in perspective, it becomes evident that a wide variety of functionalized polyolefins can be synthesized by using controlled monomer design, some of which are illustrated in Fig. 2. 

In the effort to make pure blue-emitting materials Shim and coworkers [146] synthesized a series of PPV-based copolymers containing carbazole (polymers 95 and 96) and fluorene (polymers 97 and 98) units via Wittig polycondensation. The use of trimethylsilyl substituents, instead of alkoxy groups, eliminates the electron donor influence of the latter and leads to chain distortion that bathochromically shifts the emission (Amax = 480 nm for 95 and 495 nm for 97). In addition, a very high PLQY was found for these polymers in the solid state (64 and 81%, respectively). Single-layer PLEDs fabricated with 95 and 97 (ITO/polymer/Al) showed EL efficiencies of 13 and 32 times higher than MEH-PPV, respectively (see also Ref. [147] for synthesis and PLED studies of polymers 99 and 100) (Chart 2.20). 

All reactions involved in polymer chain growth are equilibrium reactions and consequently, their reverse reactions lead to chain degradation. The equilibrium constants are rather small and thus, the low-molecular-weight by-products have to be removed efficiently to shift the reaction to the product side. In industrial reactors, the overall esterification, as well as the polycondensation rate, is controlled by mass transport. Limitations of the latter arise mainly from the low solubility of TPA in EG, the diffusion of EG and water in the molten polymer and the mass transfer at the phase boundary between molten polymer and the gas phase. The importance of diffusion for the overall reaction rate has been demonstrated in experiments with thin polymer films [10]. 

According to reports Po et al. [31] and Amoco [32], the reaction rate of PEN is lower than other polyesters. Considerations about this fact lead to the assumption that the structure-dependent reactivities of the acid and glycol components and their mobilities are responsible for the individual reaction rates of these polymers. Based on unpublished data, rigid or voluminous co-monomers result in reduced reactivities during melt polycondensation and SSP. The mobility of the component, as a result of its structure and stiffness, seems to explain this observation. 

Polycondensation of Bisphenols, II, with Phosgene. Polycondensation of siloxane-linked bisphenols, II, with phosgene is the most obvious synthetic approach leading to siloxane-modified poly(arylene carbonates) since the phosgene-bisphenol polycondensation is used in the synthesis of aromatic polycarbonates (1). This method was used initially to prepare polymer (as indicated in reaction 1) as well as for the attempted synthesis of polymers 2 and 5 ... 

These reactions were run under the same PTC conditions as those with HFB. In every case, the bisaryls (4a-c) underwent 4,4 disubstitution with 2 equivalents of nucleophile to give excellent, often quantitative yields of 5a-c as the only detectable ether or sulfide product. When excess nucleophile was employed, again only 5a-c were produced and no polysubstituted products were detected, which indicated that PTC polycondensation of bisaryls via aromatic substitution should be facile and lead to linear, branch free polymers. This proved to be the case. 

The key to a controlled molecular weight build-up, which leads to the control of product properties such as glass transition temperature and melt viscosity, is the use of a molar excess of diisopropanolamine as a chain stopper. Thus, as a first step in the synthesis process, the cyclic anhydride is dosed slowly to an excess of amine to accommodate the exothermic reaction and prevent unwanted side reactions such as double acylation of diisopropanolamine. HPLC analysis has shown that the reaction mixture after the exothermic reaction is quite complex. Although the main component is the expected acid-diol, unreacted amine and amine salts are still present and small oligomers already formed. In the absence of any catalyst, a further increase of reaction temperature to 140-180°C leads to a rapid polycondensation. The expected amount of water is distilled (under vacuum, if required) from the hot polymer melt in approximately 2-6 h depending on the anhydride used. At the end of the synthesis the concentration of carboxylic acid groups value reaches the desired low level. 

This type of polymerisation generally involves a repetitive condensation reaction between two bl-functlonal monomers. These polycondensation reactions may result in the loss of some simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation polymers. 

A polymer derived from the polycondensation of a single actual monomer, the molecules of which terminate in two different complementary functional groups (e.g. 6-aminohexanoic acid) is, by definition, a (regular) homopolymer. When two different monomers of this type react together, the product is a copolymer that can be named in appropriate fashion. For example, if 6-aminohexanoic acid is copolycondensed with 7-aminoheptanoic acid, leading to a statistical distribution of monomeric units, the product is named poly[(6-aminohexanoic acid)-stoi-(7-aminoheptanoic acid)]. 

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